The present invention relates to a heat treatment for alumina ceramic composite, particularly to a heat treatment for particle-reinforced alumina ceramic composites.
Alumina is a popular material used in industries, since it has some excellent properties, such as high strength, high hardness and high corrosion resistance. However, it still has problems of lack of toughness and low reliability preventing it from wider applications. Many people, therefore, have tried to increase its toughness and reliability by making alumina-based composite.
Conventionally, there are two methods for making ceramic composite, i.e., adding fiber (or short fiber), or adding particle as reinforcement. Experiments have shown that fiber will toughen and strengthen properties, but it is difficult to obtain an fully densified ceramic composite. The particle reinforced ceramic composite, on the other hand, can be sintered to an full densified ceramic body, yet its mechanical properties are not so good as the fiber reinforced one.
In general, the process of making particle reinforced ceramic composite is more convenient and less expensive than that of fiber/whisker reinforced ceramic composite alumina composite. For example, the sintering temperature for alumina composite is lowered from 1850.degree. C. (Al.sub.2 O.sub.3 /SiCw, added with silicon carbide whisker) to 1400.degree. C. (Al.sub.2 O.sub.3 /Cr.sub.3 C.sub.2 P, added with chromium carbide particles), therefore the latter process saves a lot of energy. The use of ceramic whisker or short fiber may carcinogenic, so ceramic particle is move acceptable to the operators. In addition, the price of ceramic powder is much lower than that of ceramic fiber or whisker; also the mechanical properties of particle reinforced ceramic composite is anisotropic so it has wider applications.
According to some Japanese and U.S. patents, the processing steps used and described in Japan Patents, No: 02-124761, No: 02-141467, No: 02-229758, No: 02-279558, No: 02-255561 and No: 61-286272, and U.S. Pat. No: 4,789,277 and U.S. Pat. No. 4,657,877 are almost the same (as shown in FIG. 1). However, the reinforcements added to each of these ceramic composites are different. In summary, the main kind of reinforcements used in the composites and their contents are: A. silicon carbide whiskers (10-40 vol %); B. zirconia particles (7-35 wt %); C. sintering agent for alumina (0.7-7 wt %), i.e., more than one kind selected from CaO, MgO, SiO.sub.2, NiO or Y.sub.2 O.sub.3 etc.; D. sintering agent for silicon carbide (0.05-5 wt %), e.g., B, C, AIN and B.sub.4 C etc.; E. particulate carbide, boride or nitride of an element selected from groups IVA, VA or VIA of periodic table (5-30 wt %); and F. the ceramic composite is balanced stoichiometrically using alumina (Al.sub.2 O.sub.3). Although the strength and the toughness of ceramic composites with particulate reinforcement is much higher than the single phase ceramic monolith, there are still some disadvantages existed as will be described in the following paragraphs.
For the combination of B+F, the toughness has been increased greatly due to the phase transformation of ZrO.sub.2. However, this advantage will be diminished at high temperatures, especially when T&gt;1100.degree. C. The hardness of the composite is lowered due to the addition of ZrO.sub.2.
For the combination of A+F or A+E+F, whisker or whisker/particles is used to reinforce ceramic composites. However, a high sintering temperature is required for obtaining high density as well as high toughness, for example, the hot-pressing temperature usually has to be greater than 1800.degree. C.
Combination A+C+D+E+F, in which additional sintering agents for Al.sub.2 O.sub.3 and SiC are used, it still requires a high hot-pressing temperature, i.e., higher than 1650.degree. C., to produce an full densified ceramic body. An adverse effect is difficult to homogenize so many additives and defects may appear within the composites thus consequentially may lower the strength and toughness.
For combination of E+F, although densified ceramic composites can be produced at a lower temperature (higher than 1300.degree. C.), the strength and toughness of the product are not high enough.
As a result, the addition of ceramic particles of carbide, boride and nitride particles in alumina can lower its sintering temperature. Yet the strengthening and toughening effects are low, so the application of the particulate reinforced alumina is restricted.